total of all given sides:
3x^4 - 4 + 4x^3 + 6x + 4x^4 -2x^2 - 3x + 5x^3 - 2x^2
= 7x^4 + 9x^3 - 4x^2 + 3x - 4
missing side = perimeter - total given sides
missing side = (7x^4 + 9x^3 - 6x^2 + 10) - (7x^4 + 9x^3 - 4x^2 + 3x - 4)
missing side = 7x^4 + 9x^3 - 6x^2 + 10 - 7x^4 -9x^3 + 4x^2 - 3x + 4
missing side = - 2x^2 - 3x + 14
answer
- 2x^2 - 3x + 14